The present invention relates to pressure-sensitive adhesive sheets for dicing which are used for fixing a material to be cut such as a semiconductor wafer in cutting and separating (dicing) a semiconductor wafer or the like into small element pieces.
Wafers of a semiconductor such as silicon, gallium or arsenic are produced so as to have a large diameter, cut and separated (diced) into small element pieces, and then sent to a mounting step. In this procedure, such a semiconductor wafer in the sate of adhering to a pressure-sensitive adhesive sheet is subjected to the steps of dicing, cleaning, expanding, pickup, and mounting. For the steps of from semiconductor wafer dicing to pickup, a pressure-sensitive adhesive sheet obtained by applying a pressure-sensitive adhesive to a substrate comprising a plastic film has been used.
In the dicing step, the wafer is cut with a rotating disk blade. The cutting technique which is coming to be mainly used for this dicing is the technique called full cutting, in which cutting is conducted to an inner part of the pressure-sensitive adhesive sheet maintaining the wafer. In the full cutting, since cutting is conducted up to an inner part of the sheet, the plastic film itself comes into a molten state due to its frictional heat to generate a fibrous waste of the plastic film itself on the dicing line after dicing. When this fibrous waste adheres to side walls or other parts of the chips, a problem arises that the adhered fibrous waste is mounted and encapsulated as it is in later steps to considerably reduce the reliability of the resulting semiconductor elements.
Means for eliminating such a problem have been proposed. For example, JP-A-5-156214 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) proposes using an ethylene/methacrylate copolymer as a substrate. Although this technique is effective in reducing the generation of a fibrous waste in some degree, the diminution of waste generation attainable with this technique is insufficient for the dicing which will be required for highly reliable semiconductors. JP-A-5-211234 proposes using a substrate film which has been irradiated with a radiation, e.g., electron beams or xcex3-rays, in a dose of from 1 to 80 Mrad. However, this technique is undesirable in quality and cost because not only the irradiation causes considerable film damage, making it difficult to obtain a film having a satisfactory appearance, but also the film production is highly costly. Furthermore, JP-A-11-43656 proposes a pressure-sensitive adhesive sheet comprising a non-stretched polypropylene layer and a pressure-sensitive adhesive layer formed thereon. However, since the non-stretched polypropylene substrate tends to tear during expansion, the proposed pressure-sensitive adhesive sheet can be used only under expansion conditions such that substrate tearing does not occur. The proposed technique is hence not sufficiently suitable for general use in dicing.
The present invention has been made to overcome the problems of conventional techniques described above.
Accordingly, one object of the present invention is to provide a pressure-sensitive adhesive sheet for dicing which can prevent the generation of a fibrous waste during dicing without having a decrease in product quality or a disadvantage in cost.
Another object of the present invention is to provide a pressure-sensitive adhesive sheet for dicing which not only can prevent the generation of a fibrous waste during dicing but has flexibility which enables the pressure-sensitive adhesive sheet to fully cope with various expansion conditions.
As a result of intensive studies in order to accomplish the above objects, it has been found that the generation of a fibrous waste during dicing can be prevented by using a substrate film constituted of a specific polymer blend comprising two or more polymers. The present invention has been completed based on this finding.
The present invention provides a pressure-sensitive adhesive sheet for dicing which comprises a substrate film and formed thereon a pressure-sensitive adhesive layer for adhesively fixing a material to be diced, wherein the substrate film has a single- or multilayer structure and at least a layer in the substrate film, which contacts the pressure-sensitive adhesive layer comprises a semi- or non-compatibilized polymer blend comprising two or more polymers. Hereinafter, this pressure-sensitive adhesive sheet for dicing is sometimes referred to as a xe2x80x9cpressure-sensitive adhesive sheet 1xe2x80x9d.
In the preferred embodiment of the pressure-sensitive adhesive sheet 1, at least one of the polymers constituting the polymer blend is a highly crystalline olefin resin having a crystallinity of 40% or higher and the content of this polymer in the polymer blend is 1 to 50% by weight based on the weight of the polymer blend. Further, at least one of the polymers constituting the polymer blend is a lowly crystalline olefin resin having a crystallinity lower than 40% and the content of this polymer in the polymer blend is 50 to 99% by weight based on the weight of the polymer blend.
The present invention further provides a pressure-sensitive adhesive sheet for dicing which comprises a substrate film and formed thereon a pressure-sensitive adhesive layer for adhesively fixing a material to be diced, wherein the substrate film has a single- or multilayer structure and at least a layer in the substrate film, which contacts the pressure-sensitive adhesive layer comprises a polymer blend comprising a lowly crystalline polymer having a crystallinity lower than 40% and a highly crystalline polymer having a crystallinity of 40% or higher. Hereinafter, this pressure-sensitive adhesive sheet is sometimes referred to as a xe2x80x9cpressure-sensitive adhesive sheet 2xe2x80x9d.
In the preferred embodiment of the pressure-sensitive adhesive sheet 2, the contents of the lowly crystalline polymer and the highly crystalline polymer in the polymer blend are 50 to 99% by weight and 1 to 50% by weight, respectively.
Further, in the pressure-sensitive adhesive sheet 1 or 2, the pressure-sensitive adhesive layer may be, for example, a radiation-curable pressure-sensitive adhesive layer.
The mechanism of the generation of a fibrous waste during dicing is presumed to be probably as follows. In dicing a wafer or the like, a blade (disk blade) which is rotating at a speed as high as usually 30,000 rpm or higher is used to dice the wafer together with a surface part of a tape (pressure-sensitive adhesive sheet). In this dicing, a surface part of the substrate film is cut while coming into a molten state due to the heat of friction with the blade rotating at a high speed. As shown in FIG. 1, this molten substrate resin is flung up by the blade and then solidifies upon cooling with cutting/cooling water in a dicer, resulting in the generation of a fibrous waste.
According to the present invention, since the substrate film comprises a specific polymer blend as described above, the polymers tend to separate from each other to form microscopic particles, when the substrate resin melts and the molten resin is flung up. This is because the bonding strength between the polymers is low and the polymers differ from each other in melt viscosity, melting point, etc. Thus, the generation of a fibrous waste is significantly inhibited.